Flame in an animation
I think you need to "bake your simulation"
It sounds like you are recalculating it every single time you play it
and your computer isnt fast enough to calculate the animation in real time

But if you bake it, it will save the calculations so it wont have to 
re-calculate it all the time. 

https://www.blender.org/manual/physics/smoke/baking.html